This invention relates to aqueous polyurethane compositions and to films made from such compositions. The invention also relates to yellowing resistant floor coverings.
Large numbers of polyurethane coating compositions are known. Many of these unfortunately are solvent solutions or dispersions rather than aqueous solutions or dispersions. Solvents are often used in such compositions because of improved solubility and better water resistance in a coated film. Such systems which are moisture cured, for example, are described in U.S. Pat. Nos. 3,929,726; 4,017,493 and 4,436,784. U.S. Pat. No. 3,929,726, for example, describes solvent based color stabilized polyurethanes containing organic hydrazides (not hydrazine). U.S. Pat. Nos. 4,017,493 and 4,436,784 discloses moisture curable polyurethanes for coating vinyl substrates.
Unfortunately, the advantages of solvent based systems are offset by disadvantages, i.e., high solvent cost and pollution.
Attempts were therefore made to make a water dilutable polyurethane, which would have the desirable characteristics of the solvent systems, i.e. high solution solids, good film strength and good water resistance.
Such attempts have been more or less successful, e.g. as described in such patents as U.S. Pat. Nos. 4,764,553; 4,652,466; 4,594,385; 4,147,679; 4,447,571; 4,269,748; 4,046,729 and 4,066,591. All of such water dispersed compositions nevertheless have had at least some disadvantages.
For example, the dispersions might not be as stable as desired, films formed from the dispersions may be too water sensitive or other physical properties may not be a good as desired; the products are made from materials that are costly or difficult to manufacture; undesirable leachable additives such as surfactants are employed and yellowing characteristics at curing temperatures are not as good as desired.
Some of such materials are water-based colloidal anionic polyurethanes, made by diamine chain extension of aqueous dispersions of isocyanate prepolymers containing salt groups which are generally tertiary amine carboxylates where the carboxylate group is on the prepolymer backbone. Polyurethanes of this type may be used in conjunction with melamineformaldehyde curing agents in topcoats for vinyl flooring. These coating compositions typically exhibit yellowing when baked on the vinyl at temperatures in the range of 180-200.degree. C.
Even when the term "non-yellowing" is used in conjunction with such materials, e.g. as described in the foregoing patents, the term is relative since yellowing characteristics are still not as good as desired.